This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission. The present invention relates to the marking of sand for oceanographic studies. These studies provide useful information in oil well operations, waste disposal operations, ecological studies, and other studies where it is important to analyze the motion of particles in water. In such studies it is important that the sand be tagged in such a manner that its hydraulic properties are not affected. It is also important that the radioactive tag be highly resistant to leaching by salt water.
Some sediment transport studies are conducted in a geographical area where sand is moved only a few feet over a period of several months. The data from such tests is extrapolated to determine the distance the sand will move over a period of years. When this type of study is made, it is important that the half-life of the radioisotope be reasonably long, because the longer the study can be continued, the more accurate the extrapolated results will be.
The prior art includes several methods by which sand particles are tagged with radioactive material. Some processes involve attaching a radioactive material by means of a cement directly to the sand particle. Other methods use ceramic compositions to attach the radioisotope by fusion to the sand at high temperatures. Another method, disclosed in U.S. Pat. No. 3,700,602, involves the reduction of gold-198 by an aldehyde. Still another method involves neutral irradiation to transmute impurities in the sand into radioactive species; however, this method is very expensive.
A problem encountered in some methods cited is that the hydraulic properties of the sand are altered during the tagging process. Studies using such sand would be less accurate than studies using the subject invention.
Another problem with some of the earlier methods is the lack of long term stability of the tagged particle in sea water, the sea water leaching the tag out of the sand. Studies with such sand would be limited to qualitative studies of short duration.
Sand tagged with gold-198 by the process disclosed in U.S. Pat. No. 3,700,602 is stable in sea water; however, the half-life of gold-198 (64.8 hours) limits its utility to short term studies.